Myofibrillar ATPase activity and the force-pCA relationship were measured in rat cardiac preparation across a broad age range. Between 2 and 6 months, maximum ATPase activity in detergent treated myofibrils decreased approximately twofold (0.166 + or - 0.01 vs 0.078 + or - 0.02 micromolar Pi/min/mg/protein, P less than .001), but did not change with further aging (12 or 24 mo). Calcium-dependent force in thin "Triton skinned" papillary muscles were not age-related. ATPase activity and force exhibited identical calcium sensitivity from the submicromolar to micromolar range. No age differences in these parameters were observed. In the intact muscles prior to skinning, neither twitch force nor the rate of force production were age-related; however, time to peak force, half relaxation time, increased progressively, changing by approximately 30% across the age span (P less than .001). Since the decline in ATPase activity occurred over the maturational period only, and did not change with further aging, while the twitch duration changed progressively with age, it is concluded that the twitch prolongation of the senescent myocardium cannot be directly related to the age-related decline in myofibrillar ATPase activity.